Hosokawa Katsumoto, (born 1430, Japan—died June 6, 1473, Kyōto), leader of a powerful military faction in medieval Japan whose dispute with Yamana Mochitoyo, the head of the powerful Yamana clan, resulted in the Ōnin War (1467–77). This conflict ravaged the area around the capital at Kyōto and destroyed central control over the country’s outlying regions, giving rise to almost a century of internecine warfare throughout Japan.

Hosokawa was a member of a family that included the leading retainers of the Ashikaga shogunate, the hereditary military dictatorship dominating Japan, at least in name, from 1338 to 1573. As such, in 1452, Hosokawa succeeded to the post of kanrei, or shogunal prime minister, which he occupied until 1464. During this period the Yamana clan emerged as the dominant power in western Japan, and Yamana Mochitoyo began an attempt to check the power of Hosokawa within the shogunate government. In spite of the efforts of Ashikaga Yoshimasa, who served as shogun from 1449 to 1473, to restrain the two factions, the struggle between Hosokawa and Yamana finally erupted into war in 1467; when Yoshimasa decided to change his choice of heir-designate from his younger brother to his infant son, the two factions became enmeshed in the dispute.

The war dragged on inconclusively until 1477, although both Hosokawa and Yamana died in 1473. With succeeding generations, however, Hosokawa’s descendants became much more powerful than those of Yamana.

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